1. Field of the Invention
The present invention relates to a vertical glass melting furnace to afford homogenized molten glass. More particularly, it is concerned with a vertical glass melting furnace to afford highly homogenized molten glass free of bubbles and unmelted substances entrapped therein owing to an electric resistance heating element immersed in the molten glass.
2. Description of the Prior Art
There are several methods for homogenizing molten glass in a glass melting furnace. They are based on mixing by convection or gas bubbling.
There is disclosed in Japanese Patent Laid-open No. 199030/1990 a method for homogenizing molten glass in a glass melting furnace. This method is applicable to a vertical glass melting furnace having an inlet for raw materials at the top thereof, an outlet for molten glass at the bottom thereof, and at least one electric resistance heating element immersed in molten glass at some level thereof such that it covers almost entirely the cross-section of the furnace at that level. According to this method, homogenization is accomplished by the convection of molten glass which is induced by a heating element for homogenization installed in the furnace.
In the case of homogenization by forced circulation with a stirrer, it is common practice to install a homogenizing tank separately from the melting furnace, thereby eliminating entrapment of bubbles and unmelted substances in molten glass by stirring.
There is a different situation in a vertical glass melting furnace of the type in which raw materials cover the surface of molten glass (forming so-called cold top) so as to reduce heat loss from the top of the furnace and the melting of raw materials is accomplished by applying an electric current directly to the molten glass. In this case, efforts to promote homogenization of molten glass by vigorous convection causes with entrapment of bubbles and unmelted substances in molten glass. Thus, with such a glass melting furnace, it is difficult to produce homogenized glass free of entrapped bubbles and unmelted substances. The same difficulties as above are also encountered in the case of forced circulation by a stirrer.